Veterinarian and researcher, he is committed to the science of reproduction and genetics as key tools in the conservation of domestic and wild species.

Carlos Iglesias Pastrana

DVM. MSc. PhD. Candidate to Quantitative Genetics.

The impact of human activities on natural animal populations leads to adaptive changes in their morphological and/or behavioural traits which, in certain cases, may result in effects antagonistic to natural and sexual selection. In addition, the demographic and genetic structure of the population is seriously affected, with significant ecological and evolutionary repercussions in the medium and long term.

In the case of elephants, adult males are the animals most sought after by poachers as they are the population group that develops the largest tusks compared to juveniles and females, mainly due to differential gene expression mechanisms. Chiyo et al. (2015) found that African elephants in southern Kenya that had survived poaching episodes in the same region between the 1970s and 1980s had experienced a significant reduction (22% in males and 37% in females) in mean tusk length compared to a historical reference population (1966-1968) in the same location. A reduction of 8% for males and 11% for females was also found for the character mean tusk circumference at the level of its insertion point on the animal's lip.

These authors observed homogeneity in the metric characteristics of tusks between elephants belonging to the same social group. Considering that the genetic similarity between individuals of the same social group is, in most cases, of considerable magnitude, it could be expected that the character "tusk size" has a heritable component.

In the same line of research, Raubenheimer and Miniggio (2016) provide the theoretical basis for the potential impact of illegal ivory trafficking on genomic information and phenotypic expression of agenesis or lack of tusk development in affected elephant populations. The death of individuals with larger tusks means that animals with smaller tusks or no tusks at all are included in the breeding herd.

Ultimately, this reproductive strategy ends up increasing the frequency of expression of the phenotypic trait "agenesis" as a consequence of the progressive loss of heterozygosity and variation in allele frequencies, a very common condition in fragmented and small populations.

The most recent study in this applied field (Campbell-Staton et al., 2021) has assessed the evolutionary response of the African savannah elephant population in Gorongosa National Park (Mozambique) to the intensity of poaching practices during the Civil War in this country (1992-1997). In general terms, the results obtained certify the negative impact that these practices have had on the local demography and genetic constitution of savannah elephants: a drastic reduction in population size associated with an increase in the frequency of phenotypes lacking tusks. The authors also propose a sex-linked (X chromosome) dominant inheritance for this phenotypic trait, which is lethal for males.

The direct consequences and implications of this reduction in tusk size and/or tusk agenesis in elephants are being studied by numerous multidisciplinary research teams internationally. Seeking to identify whether this phenotypic change could represent a biological advantage that guarantees their survival in the medium and long term, the academics concerned are analysing the behavioural patterns of elephants with these modified phenotypic traits at both the intraspecific and social group level, as well as in their role as a keystone species in the African ecosystem. 

References

Campbell-Staton, S. C., Arnold, B. J., Gonçalves, D., Granli, P., Poole, J., Long, R. A., & Pringle, R. M. (2021). Ivory poaching and the rapid evolution of tusklessness in African elephants. Science, 374(6566), 483-487.

Chiyo, P. I., Obanda, V., & Korir, D. K. (2015). Illegal tusk harvest and the decline of tusk size in the African elephant. Ecology and Evolution, 5(22), 5216-5229.

Raubenheimer, E. J., & Miniggio, H. D. (2016). Ivory harvesting pressure on the genome of the African elephant: a phenotypic shift to tusklessness. Head and neck pathology, 10(3), 332-335.

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